Biotic Ligand Research Articles

Fate, subcellular distribution and biological effects of rare earth elements in a freshwater bivalve under complex exposure

Rare earth elements (REE) are emerging contaminants due to their increased use in diverse applications including cutting-edge and green-technologies. Their environmental concerns and contradicting results concerning their biological effects require an extensive understanding of REE ecotoxicology. Thus, we have studied the fate, bioaccumulation and biological effects of three representative REE, neodymium (Nd), gadolinium (Gd) and ytterbium (Yb), individually and in mixture, using the freshwater bivalve Corbicula fluminea. The organisms were exposed for 96 h at 1 mg L−1 REE in the absence and presence of dissolved organic matter (DOM) reproducing an environmental contamination. Combined analysis of the fate, distribution and effects of REE at tissue and subcellular levels allowed a comprehensive understanding of their behaviour, which would help improving their environmental risk assessment. The bivalves accumulated significant concentrations of Nd, Gd and Yb, which were decreased in the presence of DOM likely due to the formation of REE-DOM complexes that reduced REE bioavailability. The accumulation of Nd, Gd and Yb differed between tissues, with gills > digestive gland ≥ rest of soft tissues > hemolymph. In the gills and in the digestive gland, Nd, Gd and Yb were mostly (>90 %) distributed among metal sensitive organelles, cellular debris and detoxified metal-rich granules. Gadolinium, Yb and especially Nd decreased lysosome size in the digestive gland and disturbed osmo- and iono-regulation of C. fluminea by decreasing Na concentrations in the hemolymph and Ca2+ ATPase activity in the gills. Individual and mixed Nd, Gd and Yb exhibited numerous similarities and some differences in terms of fate, accumulation and biological effects, possibly because they have common abiotic and biotic ligands but different affinities for the latter. In most cases, individual and mixed effects of Nd, Gd, Yb were similar suggesting that additivity approach is suitable for the environmental risk assessment of REE mixtures.

Recommended updates to the USEPA Framework for Metals Risk Assessment: Aquatic ecosystems.

In 2007, the USEPA issued its "Framework for Metals Risk Assessment." The framework provides technical guidance to risk assessors and regulators when performing human health and environmental risk assessments of metals. This article focuses on advances in the science including assessing bioavailability in aquatic ecosystems, short- and long-term fate of metals in aquatic ecosystems, and advances in risk assessment of metals in sediments. Notable advances have occurred in the development of bioavailability models for assessing toxicity as a function of water chemistry in freshwater ecosystems. The biotic ligand model (BLM), the multiple linear regression model, and multimetal BLM now exist for most of the common mono- and divalent metals. Species sensitivity distributions for many metals exist, making it possible for many jurisdictions to develop or update their water quality criteria or guidelines. The understanding of the fate of metals in the environment has undergone significant scrutiny over the past 20 years. Transport and toxicity models have evolved including the Unit World Model allowing for estimation of concentrations of metals in various compartments as a function of loading and time. There has been significant focus on the transformation of metals in sediments into forms that are less bioavailable and on understanding conditions that result in resolubilization or redistribution of metals in and from sediments. Methods for spiking sediments have advanced such that the resulting chemistry in the laboratory mimics that in natural systems. Sediment bioavailability models are emerging including models that allow for prediction of toxicity in sediments for copper and nickel. Biodynamic models have been developed for several organisms and many metals. The models allow for estimates of transport of metals from sediments to organisms via their diet as well as their water exposure. All these advances expand the tool set available to risk assessors. Integr Environ Assess Manag 2024;20:924-951. © 2023 The Authors. Integrated Environmental Assessment and Management published by Wiley Periodicals LLC on behalf of Society of Environmental Toxicology & Chemistry (SETAC).

Acute Toxicity of Copper to Three Species of Pacific Salmon Fry in Water with Low Hardness and Low Dissolved Organic Carbon.

Proposed development of a mine within Alaska's Bristol Bay watershed (USA) has raised concerns about the potential impact of copper (Cu) on Pacific salmon (Oncorhynchus spp.). We conducted 96-h flow-through bioassays using low-hardness and low dissolved organic carbon water to determine the acute lethal toxicity of Cu to sockeye (Oncorhynchus nerka), Chinook (Oncorhynchus tshawytscha), and coho salmon (Oncorhynchus kisutch) fry. We aimed to determine Cu toxicity under field-relevant water quality conditions and to assess three methods of calculating ambient Cu criteria: the biotic ligand model (BLM), a multiple linear regression model endorsed by the US Environmental Protection Agency, and the hardness-based model currently used by the State of Alaska. The criteria generated by all models were below 20% lethal Cu concentrations by factors ranging from 2.2 to 54.3, indicating that all criteria would be protective against mortality. The multiple linear regression-based criteria were the most conservative and were comparable to BLM-based criteria. The median lethal concentrations (LC50s) for sockeye, Chinook, and coho were 35.2, 23.9, and 6.3 µg Cu/L, respectively. We also used the BLM to predict LC50s for each species. Model predictions differed from empirical LC50s by factors of 0.7 for sockeye and Chinook salmon, and 1.1 for coho salmon. These differences fell within the acceptable range of ±2, indicating the model's accuracy. We calculated critical lethal Cu accumulation values for each species to account for differing water chemistry in each bioassay; the present study revealed that coho salmon were most sensitive to Cu, followed by sockeye and Chinook salmon. Our findings underscore the importance of considering site- and species-specific factors when modeling Cu toxicity. The empirical data we present may enhance Cu risk assessments for Pacific salmon. Environ Toxicol Chem 2023;42:2440-2452. © 2023 SETAC.

Development of a Machine Learning Model to Estimate the Biotic Ligand Model-Based Predicted No-Effect Concentrations for Copper in Freshwater.

The copper (Cu) biotic ligand model (BLM) has been used for ecological risk assessment by taking into account the bioavailability of Cu in freshwater. The Cu BLM requires data for many water chemistry variables, such as pH, major cations, and dissolved organic carbon, which can be difficult to obtain from water quality monitoring programs. To develop an optimized predicted no-effect concentration (PNEC) estimation model based on an available monitoring dataset, we proposed an initial model that considers all BLM variables, a second model that requires variables excluding alkalinity, and a third model using electrical conductivity as a surrogate for the major cations and alkalinity. Furthermore, deep neural network (DNN) models have been used to predict the nonlinear relationships between the PNEC (outcome variable) and the required input variables (explanatory variables). The predictive capacity of DNN models was compared with the results of other existing PNEC estimation tools using a look-up table and multiple linear and multivariate polynomial regression methods. Three DNN models, using different input variables, provided better predictions of the Cu PNECs compared with the existing tools for the following four test datasets: Korean, United States, Swedish, and Belgian freshwaters. Consequently, it is expected that Cu BLM-based risk assessment can be applied to various monitoring datasets, and that the most applicable model among the three different types of DNN models could be selected according to data availability for a given monitoring database. Environ Toxicol Chem 2023;00:1-13. © 2023 SETAC.

Effect of dissolved organic matter on copper bioavailability to a coastal dinoflagellate at environmentally relevant concentrations

The speciation and bioavailability of copper (Cu) in the marine environment are affected by the presence of dissolved organic matter (DOM). Previous studies conducted at dissolved Cu concentrations >100 nM confirmed that Cu bioavailability depends on the concentration of labile Cu, as measured by anodic stripping voltammetry (ASV), which aligns with the expectations of the biotic ligand model (BLM). However, ambient Cu concentrations in coastal waters are generally lower, ranging between 1 and 80 nM, and the effect of DOM on the bioavailability of Cu to marine organisms has not been tested within that range of Cu concentrations. The present study aims to assess the impact of two types of DOM, a commercially available fulvic acid, and marine DOM extracted by ultrafiltration, on Cu bioavailability to phytoplankton using short-term 65Cu internalisation by the marine dinoflagellate Prorocentrum micans. Results showed that Cu internalisation decreases with DOM additions as expected according to the BLM and in agreement with ASV measurements of labile Cu, at the highest tested Cu concentration (100 nM). On the contrary, at a lower Cu concentration (20 nM), organic complexes appear to be partially bioavailable, thereby challenging the general applicability of the BLM model at environmentally relevant concentrations in coastal areas.

Modeling the Interaction and Uptake of Cd-As(V) Mixture to Wheat Roots Affected by Humic Acids, in Terms of root cell Membrane Surface Potential (ψ0).

The joint toxicological effects of Cd2+ and As(V) mixture on wheat root as affected by environmental factors, such as pH, coexisting cations, and humic acids etc., were investigated using hydroponic experiments. The interaction and toxicological mechanisms of co-existing Cd2+ and As(V) at the interface of solution and roots in presence of humic acid were further explored by incorporating root cell membrane surface potential ψ0 into a mechanistic model of combined biotic ligand model (BLM)-based Gouy-Chapman-Stern (GCS) model and NICA-DONNAN model. Besides, molecular dynamics (MD) simulations of lipid bilayer equilibrated with solution containing Cd2+ and H2AsO4- further revealed the molecular distribution of heavy metal(loid) ions under different membrane surface potentials. H2AsO4- and Cd2+ can be adsorbed on the surface of the membrane alone or as complexes, which consolidate the limitation of the macroscopic physical models.

Implementing approaches to account for metal bioavailability in freshwaters: Current status and future directions.

Implementing approaches to account for metal bioavailability in freshwaters: Current status and future directions.

<i>Corrigendum to</i>: Cadmium thiosulfate complexes can be assimilated by a green alga via a sulfate transporter but do not increase Cd toxicity

<sec> Environmental context Thiosulfate is present in natural waters, especially those influenced by sulfide oxidation, and it has a marked affinity for metals such as cadmium. Normally the binding of cadmium by thiosulfate would be expected to reduce the metal’s bioavailability. However, here we demonstrate that algal uptake of cadmium is enhanced in the presence of thiosulfate, indicating that Cd can enter the alga via a novel route as an intact Cd-thiosulfate complex. </sec> <sec> Rationale For a given free metal ion activity in the exposure solution, the Biotic Ligand Model assumes that metal uptake will be independent of the various ligands present in solution that are buffering [M<sup><i>z</i>+</sup>]. In this context, we have evaluated cadmium bioavailability in the absence or presence of thiosulfate, using <i>Chlamydomonas reinhardtii</i> as the test alga. </sec> <sec> Methodology Short-term exposures (≤41 min) were run with a fixed concentration of the free Cd<sup>2+</sup> ion (3.0 ± 0.1 nM), buffered with either nitrilotriacetate or thiosulfate, to determine Cd uptake. Subsequent long-term exposures (72 h) over a range of free Cd<sup>2+</sup> concentrations were used to determine the effects of Cd on algal growth. </sec> <sec> Results Contrary to Biotic Ligand Model predictions, Cd uptake was enhanced when Cd<sup>2+</sup> was buffered with thiosulfate. Removal of sulfate from this exposure medium increased Cd uptake; conversely, if [SO<sub>4</sub><sup>2−</sup>] was increased, Cd uptake decreased. In the absence of thiosulfate, Cd uptake was unaffected by changes in [SO<sub>4</sub><sup>2−</sup>]. In the long-term exposures, the cellular Cd quota needed to reduce algal growth by 50% was significantly higher in the presence of thiosulfate than in its absence. </sec> <sec> Discussion In the presence of thiosulfate, Cd can enter the algal cell not only by cation transport but also by transport of the intact Cd-thiosulfate complex via the anion transporter responsible for sulfate uptake. We speculate that some of the Cd taken up by anion transport remains in complexed form and is less bioavailable than the Cd that enters the cell via cation transport. </sec>

Validation of Nickel Bioavailability Models for Algae, Invertebrates, and Fish in Chinese Surface Waters.

Nickel (Ni) is used primarily in the production of alloys like stainless steel and is increasingly being used in the production of batteries for the electric vehicle market. Exposure of Ni to ecosystems is of concern because Ni can be toxic to aquatic organisms. The influence of water chemistry constituents (e.g., hardness, pH, dissolved organic carbon) on the toxicity of Ni has prompted the development and use of bioavailability models, such as biotic ligand models (BLMs), which have been demonstrated to accurately predict Ni toxicity in broadly different ecosystems, including Europe, North America, and Australia. China, a leading producer of Ni, is considering bioavailability-based approaches for regulating Ni emissions. Adoption of bioavailability-based approaches in China requires information to demonstrate the validity of bioavailability models for the local water chemistry conditions. The present study investigates the toxicity of Ni to three standard test species (Daphnia magna, Pseudokirchneriella subcapitata, and Danio rerio) in field-collected natural waters that are broadly representative of the range of water chemistries and bioavailabilities encountered in Chinese lakes and rivers. All experimental data are within a factor of 3 of the BLM predicted values for all tests with all species. For D. magna, six of seven waters were predicted within a factor of 2 of the experimental result. Comparison of experimental data against BLM predictions shows that the existing Ni bioavailability models are able to explain the differences in toxicity that result from water chemistry conditions in China. Validation of bioavailability models to water chemistries and bioavailability ranges within China provides technical support for the derivation of site-specific Ni water quality criteria in China. Environ Toxicol Chem 2023;42:1257-1265. © 2023 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

Contaminant curiosity and pollutant puzzles: Conceptual insights in ecotoxicity and practical implementation of higher-tiered risk assessment.

Abstract Since the soil quality Tool for Risk Identification, Assessment and Display (TRIAD) approach introduced the “three lines of evidence” accounting for chemical, toxicological and ecological stressors to explain adverse effects in biota, the assessment of contaminant risks in the environment has significantly evolved. The concept of chemical speciation, related to water characteristics, boosted the understanding of the role of free-ion activities in the overall accumulation of pollutants in biota. New modeling concepts (e.g. biotic ligand models) and measuring techniques were developed. This in turn triggered widespread research addressing the quantitative role of sediment in the overall water quality, focusing on redox interfaces. For contaminant mixtures in river catchments, complex relations between (bio)availability of compounds, including nutrients, help to explain aquatic toxicity. Variation in ecological patterns and processes across environmental or spatiotemporal gradients occur, which may identify ecological factors that influence contaminant fate and effects. Empirical evidence by meta-analysis and theoretical underpinning by modelling showed relationships between population growth rates and carrying capacities, across chemicals and across species. The potentially affected fraction of species may be related to the mean species abundance, an often-used indicator in global change studies. Knowledge gaps remain on how pollutants travel through ecological communities and which species and species-relationships are affected. Outdoor experimental systems that examine the natural environment under controlled conditions may be useful at the higher biological level to investigate the impact of stressors on a variety of species, including mutual interactions.

Comparison of the Estimation Methods from Acute to Chronic Biotic Ligand Model-Based Predicted No-Effect Concentrations for Nickel in Freshwater Species.

Biotic ligand models (BLMs) and the sensitivities of indigenous species are used to assess the environmental risk considering the bioavailability of metals, such as nickel. However, the BLM-based acute-to-chronic ratio (ACR) is required if the predicted no-effect concentration (PNEC) cannot be derived from the chronic species sensitivity distribution (SSD). The applicability of the ACR approach for estimating BLM-based PNEC for nickel from acute toxicity data was evaluated in the present study. The BLM-based acute SSD for nickel was built using the sensitivities of 21 indigenous species and different taxon-specific BLMs for each taxonomic group. To predict the acute sensitivity of invertebrates, the chronic crustacean nickel BLM with pH effect term, which can account for nickel toxicity at high pH levels, was used. This was used instead of the existing acute BLM for crustacean, which has too narrow a pH range to cover the pH dependency of toxicity. The final BLM-based ACR of nickel, determined within a factor of 1.53 from the species-specific acute and chronic sensitivities of the six species, was more reliable than the typical ACR estimated within a factor of 1.84. A linear relationship (r2 = 0.95) was observed between the PNECs using BLM-based ACR and the PNECs derived from the BLM-based chronic SSD of the European Union Risk Assessment Reports. In conclusion, the BLM-based PNEC for nickel could be derived using the ACR approach, unlike when copper BLM was applied. The BLM-based ACR for nickel is the first result calculated by directly comparing acute and chronic species sensitivities, and will contribute to the application of BLM-based risk assessment in broader ecoregions. Environ Toxicol Chem 2023;00:1-14. © 2023 SETAC.

Amelioration of cadmium cytotoxicity to human cells by nutrient cation contents and the building of a biotic ligand model

A variety of important major and trace elements may competitively inhibit cadmium (Cd) absorption in human cells and reduce Cd toxicity. However, the impact of essential elements on the cytotoxicity of metals can be difficult to quantify and anticipate. Cd acute toxicity to Caco-2 cell viability was studied in culture solutions and modeled by a biotic ligand model (BLM). The individual effects of the cations potassium (K+), calcium (Ca2+), magnesium (Mg2+), ferrous ion(Fe2+), zinc (Zn2+) and manganese (Mn2+) on Cd toxicity were also investigated. The results indicated that the toxicity of Cd in culture solutions to cell viability declined with increasing concentrations of Zn2+ and Mn2+ in the solutions, while K+, Ca2 +, Mg2 + and Fe2+ had no significant effect. Using the BLM, the stability constants for the binding of Cd2 +, Zn2+, and Mn2+ to biotic ligands were determined to be logKCdBL = 5.76, logKZnBL = 4.39 and logKMnBL = 5.31, respectively. Moreover, it was calculated that 51% occupancy of the biotic ligand sites for Cd by Cd was required to cause a 50% reduction in Caco-2 cell viability. A BLM was successfully established using the estimated constants to predict the Cd cytotoxicity to Caco-2 cell viability as a function of solution characteristics, so that the effective concentrations that reduced cell viability by 50% (EC50) could be predicted by the BLM within 1.6 fold changes of the observed EC50. The application's viability and precision for foretelling Cd toxicity in Caco-2 cells are discussed.

Updating the Chronic Freshwater Ecotoxicity Database and Biotic Ligand Model for Nickel for Regulatory Applications in Europe.

Bioavailability has been taken into account in the regulation of nickel in freshwater ecosystems in Europe for over a decade; during that time a significant amount of new information has become available covering both the sensitivity of aquatic organisms to nickel toxicity and bioavailability normalization. The ecotoxicity database for chronic nickel toxicity to freshwater organisms has been updated and now includes 358 individual data points covering a total of 53 different species, all of which are suitable for bioavailability normalization to different water chemistry conditions. The bioavailability normalization procedure has also been updated to include updates to the bioavailability models that enable more sensitive water chemistry conditions to be covered by the model predictions. The updated database and bioavailability normalization procedure are applicable to more than 95% of regulated European surface water conditions and have been used to calculate site-specific criteria for a variety of different water chemistry scenarios, to provide an indication of how the sensitivity to nickel varies between different water types. The hazardous concentration for 5% of a species (HC5) values for this diverse selection of water types range from 1.6 to 36 µg L-1 , clearly demonstrating the importance of accounting for nickel bioavailability in freshwaters. This updated database and bioavailability normalization procedure provide a robust basis for the derivation of regulatory thresholds for chronic nickel toxicity in freshwaters such as predicted no-effect concentrations and Environmental Quality Standards and are protective of the results of several mesocosm studies. Environ Toxicol Chem 2023;42:566-580. © 2023 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

Size-Fractionated Weathering of Olivine, Its CO2-Sequestration Rate, and Ecotoxicological Risk Assessment of Nickel Release

Olivine, one of the most abundant silicates on earth, thermodynamically captures CO2 in relevant amounts during its dissolution. Upscaling the use of this mineral as a replacement for sand or gravel may contribute to reduce concentrations of greenhouse gasses in the atmosphere. However, the reliable quantification of weathering rates and prognoses for effects of various environmental conditions on weathering are lacking. This currently inhibits the monitoring, reporting and verification of CO2 capture and hampers the exploitation of the carbon dioxide removal economy. A mineral dissolution model was developed, and olivine weathering rates were directly coupled to particle sizes of the ground mineral. A particle size-dependent calculation approach, based on the shrinking core model, showed faster weathering rates as compared to a single-size, monodisperse approach. This provided a better underpinning of the prediction of the overall weathering and, consequently, the sequestration rate of CO2. Weathering of olivine releases nickel, which is incorporated in the mineral. The dissolution model was coupled to advanced biotic ligand models (BLM) for nickel in order to assess potential chronic ecotoxicological risks upon release in the environment. Predicted no-effect concentrations for nickel showed that both the release of Mg and the increase of pH following olivine weathering significantly lowers nickel ecotoxicity.

Influence of Geochemical Fractionation of Fulvic Acid on its Spectral Characteristics and its Protection Against Copper Toxicity to Daphnia magna.

Dissolved copper (Cu) can contribute to toxicity in aquatic systems impacted by acid mine drainage (AMD), and its bioavailability is influenced by aqueous complexation with organic ligands that predominantly include fulvic acids (FAs). Because the geochemical fractionation of FAs that accompanies sorption to hydrous aluminum oxides (HAOs) and hydrous iron oxides (HFOs) can alter Cu complexation with FA, we investigated FAs isolated from three categories of water (pristine, AMD, and in situ-fractionated mixtures of pristine and AMD collected at stream confluences) in three mining-impacted alpine watersheds in central Colorado, USA. We also conducted geochemical fractionation of field-collected FAs and Suwannee River FAs by precipitating HAOs and HFOs in the laboratory. Spectral properties of the FAs (e.g., UV-VIS absorbance) were altered by geochemical fractionation, and in acute toxicity tests with an aquatic invertebrate (Daphnia magna) Cu was more toxic in the presence of in situ- and laboratory-fractionated FAs (median effect concentration [EC50] 19-50 µg Cu L-1 ) than in the presence of nonfractionated FAs (EC50 48-146 µg Cu L-1 ). After adjusting for the strain-specific sensitivity of our D. magna, we improved the accuracy of Biotic Ligand Model predictions of Cu EC50 values for AMD-related FAs by using an "effective dissolved organic carbon" based on spectral properties that account for among-FA differences in protectiveness against Cu toxicity. However, some differences remained between predicted and measured EC50 values, especially for FAs from AMD-related waters that might contain important metal-binding moieties not accounted for by our measured spectral indices. Environ Toxicol Chem 2023;42:449-462. © 2022 SETAC.

Comparison of Multiple Linear Regression and Biotic Ligand Models for Predicting Acute and Chronic Zinc Toxicity to Freshwater Organisms.

Multiple linear regression (MLR) models for predicting zinc (Zn) toxicity to freshwater organisms were developed based on three toxicity-modifying factors: dissolved organic carbon (DOC), hardness, and pH. Species-specific, stepwise MLR models were developed to predict acute Zn toxicity to four invertebrates and two fish, and chronic toxicity to three invertebrates, a fish, and a green alga. Stepwise regression analyses found that hardness had the most consistent influence on Zn toxicity among species, whereas DOC and pH had a variable influence. Pooled acute and chronic MLR models were also developed, and a k-fold cross-validation was used to evaluate the fit and predictive ability of the pooled MLR models. The pooled MLR models and an updated Zn biotic ligand model (BLM) performed similarly based on (1) R2 , (2) the percentage of effect concentration (ECx) predictions within a factor of 2.0 of observed ECx, and (3) residuals of observed/predicted ECx versus observed ECx, DOC, hardness, and pH. Although fit of the pooled models to species-specific toxicity data differed among species, species-specific differences were consistent between the BLM and MLR models. Consistency in the performance of the two models across species indicates that additional terms, beyond DOC, hardness, and pH, included in the BLM do not help explain the differences among species. The pooled acute and chronic MLR models and BLM both performed better than the US Environmental Protection Agency's existing hardness-based model. We therefore conclude that both MLR models and the BLM provide an improvement over the existing hardness-only models and that either could be used for deriving ambient water quality criteria. Environ Toxicol Chem 2023;42:393-413. © 2022 SETAC.

Quantitative ion character-activity relationship methods for assessing the ecotoxicity of soil metal(loid)s to lettuce.

New pollution elements introduced by the rapid development of modern industry and agriculture may pose a serious threat to the soil ecosystem. To explore the ecotoxicity and risk of these elements, we systematically studied the acute toxicity of 18 metal(loid)s toward lettuce using hydroponic experiments and quantitative relationships between element toxicity and ionic characteristics using ion-grouping and ligand-binding theory methods, thereby establishing a quantitative ion character-activity relationship (QICAR) model for predicting the phytotoxicity threshold of data-poor elements. The toxicity of 18 ions to lettuce differed by more than four orders of magnitude (0.05-804.44μM). Correlation and linear regression analysis showed that the ionic characteristics significantly associated with this toxicity explained only 23.8-50.3% of the toxicity variation (R2Adj = 0.238-0.503, p < 0.05). Relationships between toxicity and ionic properties significantly improved after separating metal(loid) ions into soft and hard, with R2Adj of 0.793 and 0.784 (p < 0.05), respectively. Three ligand-binding parameters showed different predictive effects on lettuce metal(loid) toxicity. Compared with the binding constant of the biotic ligand model (log K) and the hard ligand scale (HLScale) (p > 0.05), the softness consensus scale (σCon) was significantly correlated with toxicity and provided the best prediction (R2Adj = 0.844, p < 0.001). We selected QICAR equations based on soft-hard ion classification and σCon methods to predict phytotoxicity of metal(loid)s, which can be used to derive ecotoxicity for data-poor metal(loid)s, providing preliminary assessment of their ecological risks.

Effect of dissolved organic matter on the bioavailability and toxicity of cadmium in zebrafish larvae: Determination based on toxicokinetic–toxicodynamic processes

The presence of dissolved organic matter (DOM) strongly influences the bioavailability of metals in aquatic environments; however, the association between the binding activities and the concentrations of DOM compositions is not well documented, leading to uncertainties in metal toxicity assessment. We creatively quantify the mitigation and acceleration effects of DOM compositions on cadmium (Cd) bioaccumulation and toxicity in zebrafish larvae using abiotic ligand (ABLs) and biotic ligand (BLs) in a toxicokinetic–toxicodynamic (TK–TD) model. The BL–TK–TD model could accurately predict the protective effect of fulvic acid while overestimating the complexing capacity of citric acid. The model also could successfully simulate the protective effects of native DOM in most cases from 32 natural water bodies in China. The observed LC50 values of Cd showed a peak effect for the native DOM fraction comprising hydrophilic acidic contents (3.55 ± 0.44 mg L − 1) in natural water from 32 sites. The BL–TK–TD model provides practically useful information to identify the effect of different DOM compositions on metal bioavailability and toxicity in aquatic environments and guides future water management policies aimed at controlling aquatic heavy metal pollution.

Development of a Site-Specific Guideline Value for Copper and Aquatic Life in Tropical Freshwaters of Low Hardness.

Copper (Cu) is a contaminant of potential concern for a uranium mine whose receiving waters are in the World Heritage-listed Kakadu National Park in northern Australia. The physicochemical characteristics of the freshwaters in this region enhance metal bioavailability and toxicity. Seven tropical species were used to assess the chronic toxicity of Cu in extremely soft freshwater from a creek upstream of the mine. Sensitivity to Cu was as follows: Moinodaphnia macleayi > Chlorella sp. > Velesunio sp. > Hydra viridissima > Amerianna cumingi > Lemna aequinoctialis > Mogurnda mogurnda. The 10% effect concentrations (EC10s) ranged from 1.0 µg/L Cu for the cladoceran Moinodaphnia macleayi to 9.6 µg/L for the fish M. mogurnda. The EC50s ranged from 6.6 µg/L Cu for the mussel Velesunio sp. to 22.5 µg/L Cu for M. mogurnda. Geochemical modeling predicted Cu to be strongly bound to fulvic acid (80%-99%) and of low bioavailability (0.02%-11.5%) under these conditions. Protective concentrations (PCs) were derived from a species sensitivity distribution for the local biota. The 99% PC (PC99), PC95, PC90, and PC80 values were 0.5, 0.8, 1.0, and 1.5 µg/L Cu, respectively. These threshold values suggest that the current Australian and New Zealand default national 99% protection guideline value for Cu (1.0 µg/L) would not provide adequate protection in freshwaters of low hardness, particularly for this area of high conservation value. The continuous criterion concentration predicted by the Cu biotic ligand model for conditions of low pH (6.1), low dissolved organic carbon (2.5 mg/L), low hardness (3.3 mg/L), and 27 °C was 0.48 µg/L Cu, comparable with the PC99. Consideration of the natural water quality conditions of a site is paramount for protective water quality guidelines. Environ Toxicol Chem 2022;41:2808-2821. © 2022 Commonwealth of Australia. Environmental Toxicology and Chemistry © 2022 SETAC.

Investigating the mechanisms of dissolved organic matter protection against copper toxicity in fish of Amazon's black waters

We investigated how natural dissolved organic matter (DOM) of the Rio Negro (Amazon) affects acute copper (Cu) toxicity to local fish: the cardinal tetra (Paracheirodon axelrodi) and the dwarf cichlid (Apistogramma agassizii). It is established that Cu2+ complexation with DOM decreases Cu bioavailability (and thus toxicity) to aquatic organisms, as conceptualized by the Biotic Ligand Model (BLM). However, we also know that Rio Negro's DOM can interact with fish gills and have a beneficial effect on Na+ homeostasis, the main target of acute Cu toxicity in freshwater animals. We aimed to tease apart these potential protective effects of DOM against Cu-induced Na+ imbalances in fish. In the laboratory, we acclimated fish to Rio Negro water (10 mg L−1 DOC) and to a low-DOM water (1.4 mg L−1 DOC) with similar ion composition and pH (5.9). We measured 3-h Cu uptake in gills and unidirectional and net Na+ physiological fluxes across a range of Cu concentrations in both waters. Various DOM pre-acclimation times (0, 1 and 5 days) were evaluated in experiments with P. axelrodi. Copper exposure led to similar levels of net Na+ loss in the two fish, but with distinct effects on Na+ influx and efflux rates reflecting their different ionoregulation strategies. Rio Negro DOM protected against Cu uptake and toxicity in the two fish species. Both Cu uptake in fish gills and Na+ regulation disturbances were relatively well predicted by the modelled aqueous free Cu2+ ion concentration. These findings suggest that protection by DOM occurs mainly from Cu complexation under the tested conditions. The prevalence of this geochemical-type protection over a physiological-type protection agrees with the BLM conceptual framework, supporting the use of the BLM to assess the risk of Cu in these Amazonian waters.